Filter devices of this type are known, cf. Der Hydrauliktrainer, Band 1, 1991 [Hydraulics Training Manual, Volume 1, 1991], published by Mannesmann Rexroth GmbH, pp. 279 and 280. These filter devices can be designed for operation with spin-on cartridges. The filter cartridge contains a filter element permanently installed in the housing to be screwed onto the filter head. After expiration of the useful operating time of the filter element, it is replaced such that the entire cartridge with the used filter element contained therein being disposed of. On the other hand, filter devices such as these can also be operated in a design in which a filter housing interchangeably holds a filter element therein and is removably attached to the filter head so that when the filter element is changed, only the filter element is removed from the filter housing and disposed of. The filter housing provided with a new filter element continues to be used.
It is apparent that the latter design is comparatively more advantageous with respect to conservation of resources and the resulting operating costs due to the continued use of the filter housing. On the other hand, the design in which spin-on cartridges are used is characterized by especially high operating reliability, because the fixed installation of the filter element in the housing of the spin-on cartridge avoids the danger that in the course of replacement of a used filter element an empty filter housing without the fresh filter element inserted into it will be unintentionally attached to the filter head. On the other hand, in filter devices with a reusable filter housing serious damage or even destruction of the pertinent hydraulic system can occur if a filter housing without the filter element located therein is inadvertently attached to the filter head, for example through the carelessness of the operator only a used filter element being selected and insertion of a fresh element being omitted.